Abstract

Geologic mapping, petrologic and structural analyses, and U-Pb dating in the Tiefort Mountains, northern Mojave Desert, California, provide data on Paleozoic paleogeography and Mesozoic intra-arc tectonics. Metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late Precambrian Cordilleran passive-margin facies. Recognition of Precambrian augen gneiss and metasedimentary rocks with craton- margin protoliths suggests that the craton- miogeocline boundary is in or near the Tiefort Mountains; the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area. The metasedimentary units are intruded by Middle Jurassic granite and diorite orthogneisses dated by U-Pb as ca. 164 and ca. 160 Ma.

Deformed rocks record amphibolite facies metamorphism and mylonitization during Middle to Late Jurassic southeast-vergent ductile shear. Pervasively annealed microstructures indicate that recrystallization outlasted deformation. At South Tiefort Mountain, the mylonitic fabric is deformed by northwest-trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 ± 14 Ma. At Tiefort Mountain synkinematic biotite granite gneiss is dated as ca. 105 Ma, and mylonitic fabrics are folded about north-trending axes. The presence of the Cretaceous orthogneiss suggests that two deformational events, Jurassic and Cretaceous in age, may have affected part of the study area. Dikes dated as ca. 82 Ma that crosscut all fabrics are interpreted to be part of a regional intrusive event.

The timing and vergence of shear zones at South Tiefort Mountain are similar to contractional shear zones in adjacent areas, forming a belt of contractional deformation from the central Mojave Desert to the East Sierran thrust system. This deformation may be related to a period of late Middle to Late Jurassic sinistral-oblique subduction.

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